Generally speaking, the function of a surge arrester is to carry to ground excessive current on a power line resulting from lightning, sudden changes in the source or load voltage or other transient phenomena. Thus, the arrester usually has a current path to ground which includes resistance elements having special, nonlinear characteristics so that the surge is dissipated without damage to equipment on the line and so that service can continue after the transient is gone.
If a transient occurs which is too large or too long-lasting for the arrester to handle, not all of the energy can be dissipated and the arrester may then fail. It is also possible for the arrester to fail as the result of other causes such as the gradual deterioration of the resistance elements. In this context, "failure" can be defined as the breakdown of components in the arrester such that an uncontrolled or insufficiently controlled current path is established through the arrester, usually from the power line to ground.
When failure occurs, the arrester is no longer capable of performing any useful function and it is desirable to remove it from the circuit to avoid opening other circuit breakers in the system. For this purpose, arresters have been provided with disconnectors which permanently separate the ground line from the arrester. Such disconnectors include an explosive charge or other gas generating substance activated by the heat of a spark in a gap in the ground circuit. The excessive current creates enough heat to detonate the charge or otherwise generate gases the expansion of which blow a portion of the device out along a line which is intentionally constructed to be weaker than the rest of the structure, thereby physically separating the ground line from the arrester. Examples of arresters having such connectors are shown in the following U.S. Pat. Nos.:
2,957,967, MacRae; PA1 2,989,608, Hicks; PA1 3,100,246, Riley; PA1 4,503,414, Sykes et al.
In the prior art the disconnector is constructed so as to be, or to become, part of the arrester structure itself. The arrester is then supported by a separate apparatus such as a "bellyband" or bracket. The external housing leakage distance between the energized bottom end of the electrically failed arrester which is still mechanically intact and the sometimes electrically grounded bellyband bracket is designed to be sufficiently long to allow the failed arrester to remain energized without locking out the electrical system.